This invention relates generally to an improved material suitable for the outer semiconducting layer of high voltage cables having layered insulations. More specifically, the invention concerns semiconducting compositions which make possible the production of insulated cables of the character referred to above in which the insulating layer and the outer semiconducting layer are normally in a state of intimate contact with each other, and, moreover, the outer semiconducting layer can be readily stripped off in accordance with necessity.
Heretofore, in a high voltage insulated cable, semiconducting layers are formed as inner and outer layers of the insulation structure for the purpose of moderating the electric field. In order to prevent corona discharge, it is necessary that these semiconducting layers be in intimate contact with or intimate adhesion to the insulating layer in the insulation structure without any gap therebetween. However, excessive adhesion between the outer semiconducting layers and the insulating layer in the insulation structure will give rise to problems such as great difficulty in removing the outer semiconducting layer from the insulation structure in the case where, for example, two lengths of the cable are to be spliced, whereby the stripping-off work requires a long time and can cause damage to the insulating layer in the structure. Thus, cable terminal work requires much time, labor, and skill.
Semiconducting layers having ethylene-vinyl ester copolymers as their base resins, which have heretofore been considered to be the best materials for semiconducting layers of the instant kind, have the property of adhering very strongly to olefin polymers constituting the insulating layer in the insulation structures of cables. For this reason, these layers have almost no strippability from the insulating layer, whereby these semiconducting layers have made cable terminal work extremely difficult.
Representative examples of materials proposed for semiconducting layers known in the prior art are as follows.
(1) Ethylene-vinyl ester copolymers such as an ethylene-vinyl acetate copolymer (of high vinyl acetate content, hereinafter referred to as "high VAc content EVA") or ethylene-unsaturated carboxylate copolymers such as an ethylene-ethyl acrylate copolymer having carbon black dispersed therein.
(2) Halogen-containing resins such as chlorinated polyethylene, chlorosulfonated polyethylene, EVA-vinyl chloride graft copolymer, chlorinated polyethylenevinyl chloride graft copolymer, polyvinyl chloride, and chloroprene as such or blended with an olefin polymer having carbon black dispersed therein.
(3) Blends of resins such as polystyrene, styrene copolymers, butadiene-acrylonitrile copolymers, and polyester with an olefin polymer having carbon black dispersed therein.
With regard to the above materials (1), the materials lack strippability from the insulation layer as mentioned hereinbefore.
The above materials (2) have been proposed with the aim of improving the strippability thereof from the insulating layer. While their effectiveness in this respect is recognizable, they are accompanied by the following problems.
In order to suppress a dielectric breakdown phenomenon due to causes such as water tree in a cable insulated with crosslinked polyethylene, there is a trend toward changing the crosslinking system from the wet-crosslinking method to the dry-crosslinking method. In general, however, the crosslinking temperature in the dry crosslinking method is higher than that in the wet crosslinking method, whereby there is an even greater necessity for thermal stability at the time of fabrication of the outer semiconducting layer material. Under these circumstances, the halogen-containing resin, the high VAc content EVA, or the like generate corrosive gases due to thermal decomposition, at high-temperature which gases give rise to corrosion of equipment and corrosion of the shielding copper tape of the cable thereby becoming a cause of impairment of cable performance.
The above enumerated materials (3), similarly as in the case of the materials (2), are intended to improve the strippability of the semiconducting layer from the insulating layer, but in all cases, the compatibility thereof with olefin polymers is inadequate, and, moreover, there is the necessity of increasing the blend quantity of the first resin (polystyrene, for example) for improving the strippability. The resulting outer semiconducting layer thus tends to be fragile and subject to excessive peeling and breaking off of layer pieces, whereby these materials (3) are accompanied by problems in practical use.
We have studied the above described problems encountered in the prior art and have concluded that a satisfactory semiconducting resin composition for the instant purpose must fulfil the following requirements.
1. In the finished cable, the semiconducting resin composition layer must be in a state of intimate contact with the insulating layer.
2. The layer must be readily strippable from insulating layer depending on the necessity.
3. The resin composition must have excellent mechanical strength properties including flexibility and stretchability.
4. The dispersibility of the carbon black in the resin composition must be good.
5. The resin composition must have excellent moldability or extrudability.
6. The resin composition must have excellent thermal stability and must generate only a small quantity of corrosive gases when the resin composition undergoes thermal decomposition.